Unwanted changes in voltage or current within a circuit are generally referred to as a “transient.” Transients may be caused by a variety of sources. For example, lightning strikes may cause unwanted transients within a circuit. Additionally, transients may be manifested in a variety of ways. More specifically, transients may cause a number of different changes to the steady-state voltage or current conditions within a circuit. Furthermore, the current and voltage waveforms and magnitudes associated with transients also vary. As transients often cause unwanted spikes in voltage and/or current within a circuit, damage to circuit components can occur.
Protecting circuits from transients often involves placing some type of protection component (or components) between the source of potential transients and the circuit to be protected. The protection component acts to restrict the current and voltage in the circuit to safe levels. Some protection components use semiconductor devices, due to the fact that semiconductor devices often have well defined clamping voltage and leakage current characteristics.
In practice, protection components that utilize semiconductor devices may be configured to quickly limit any voltage transients to predefined levels. The capacitance of a semiconductor junction, however, changes with voltage. As such, protecting the circuit from transients may come at the expense of increased capacitance of the protection component. Some circuits, such as, for example, very high speed subscriber telephone lines (VDSL) may experience data loss due to increased capacitance levels. Accordingly, some protection components may cause data loss within the circuit being protected due to an increase in capacitance resulting from mitigating the transient.
Furthermore, the resistance of some semiconductor devices also changes with voltage. As such, protecting the circuit from transients may also come at the expense of varying resistance, which may adversely affect the current within the circuit. Additionally, some semiconductor devices may become damaged by too many cycles of power dissipation, resulting in premature failure of the protection component.
Therefore, there is a need to for protection components that are able to quickly clamp voltage of a transient to reasonable levels, sustain repeated exposure to high currents, and not substantially change the capacitance of the circuit being protected.